No effective pharmacological treatment for cocaine dependence has yet been developed. Comorbid cocaine dependence among methadone maintained patients interferes with treatment outcomes, and increases the risk of relapse to IV use, which is associated with increased risk for HIV and Hepatitis C. Enhanced GABA eurotransmission appears to attenuate cocaine's reinforcing effects. The objective of this proposal is to examine to what extend will enhancing the GABA system modify cocaine-using behavior and attenuate opiate withdrawal symptoms among newly admitted methadone treated patients. The primary hypothesis is that enhancing the GABA system with a GABA transporter blocker tiagabine we expect to reduce cocaine use and to promote abstinence in cocaine abusing methadone treated patients. The secondary hypothesis is that enhancing the GABA system with a GABA transporter blocker tiagabine we expect to attenuate early opiate withdrawal symptoms in newly admitted methadone treated patients. These hypotheses are formulated based on our pilot studies using the selective GABA re-uptake inhibitor tiagabine. This 14-week double-blind, placebo controlled randomized clinical trial will provide treatment for 120 cocaine and opioid dependent patients. Participants, aged 18-65 years, will be randomized to receive tiagabine 32mg/day, tiagabine 20mg/day or placebo while concurrently receiving treatment with methadone. All participants receive weekly 1-hour individual psychotherapy (Cognitive Behavioral Therapy) with experienced clinicians specifically trained to deliver the therapy and who will receive ongoing supervision. A follow-up at 3 months will allow us to evaluate opioid and cocaine use, and changes in psychosocial adjustment. The primary outcomes will be abstinence rates and reduction in opioid and cocaine use, as assessed by self-report and thrice-weekly urinalyses. The proposed study is expected to provide a better understanding of the role of the selective GABA re-uptake inhibitor tiagabine on cocaine using behavior and treatment of opiate withdrawal symptoms, and will provide valuable information to develop new pharmacological interventions for comorbid cocaine dependence among methadone treated patients.